The Collaborative International Dictionary of English v.0.48:

Middling \Mid"dling\, a.
   Of middle rank, state, size, or quality; about equally
   distant from the extremes; medium; moderate; mediocre;
   ordinary. "A town of but middling size." --Hallam.
   [1913 Webster]

         Plainly furnished, as beseemed the middling
         circumstances of its inhabitants.        --Hawthorne.
   [1913 Webster] -- Mid"dling*ly, adv. -- Mid"dling*ness,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

middling
    adv 1: to a moderately sufficient extent or degree; "pretty
           big"; "pretty bad"; "jolly decent of him"; "the shoes are
           priced reasonably"; "he is fairly clever with computers"
           [syn: reasonably, moderately, pretty, jolly,
           somewhat, fairly, middling, passably] [ant:
           immoderately, unreasonably]
    adj 1: lacking exceptional quality or ability; "a novel of
           average merit"; "only a fair performance of the sonata";
           "in fair health"; "the caliber of the students has gone
           from mediocre to above average"; "the performance was
           middling at best" [syn: average, fair, mediocre,
           middling]
    n 1: any commodity of intermediate quality or size (especially
         when coarse particles of ground wheat are mixed with bran)

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "middling":
   amidships, average, banal, betwixt and between, central, common,
   core, dull, equatorial, equidistant, fair, fair to middling,
   fairish, halfway, indifferent, inferior, insipid, interior,
   intermediary, intermediate, lackluster, mean, medial, median,
   mediocre, mediterranean, medium, mesial, mezzo, mid, middle,
   middle-of-the-road, middlemost, midland, midmost, midships, midway,
   moderate, modest, namby-pamby, normal, nuclear, of a kind,
   of a sort, of sorts, ordinary, passable, poor, respectable,
   routine, second-rate, so-so, standard, tedious, tolerable, usual,
   vapid, wishy-washy